1. The eruption went through several stages, with six distinct marker horizons identified. 2. The base of this marker horizon marks the formational change to the Ashdown Formation. 3. The Top Ashdown Sandstone is the most persistent marker horizon in Early Cretaceous strata throughout the region. 4. From the same series a ripple-marked quartzite and a speckled bed are used as marker horizons. 5. Marker horizons of tephra are used as a dating tool in archaeology, since the dates of eruptions are generally well-established. 6. The use of tephra layers, which bear their own unique chemistry and character, as temporal marker horizons in archaeological and geological sites is known as tephrochronology. 7. Crag Gill consists of an exposure of late Namurian limestones, sandstones and shales that form a type locality of the Whitestone limestone, a marker horizon for Namurian stratigraphy. 8. On a much smaller time scale, marker horizons may be created by sedimentologists and limnologists in order to measure deposition and erosion rates in a marsh or pond environment. 9. As on the Moon, where ejecta from the better preserved basins was used to construct a stratigraphy, the ejecta from the Caloris Basin also can be used as a marker horizon. 10. Stratigraphers and sedimentologists often use hardgrounds as marker horizons and as indicators of sedimentary hiatuses and flooding events ( F�rsich et al ., 1981, 1992; Pope and Read, 1997 ).